The International Space Station being an artificial habitat for humans; it seems reasonable to assume the air temperature, and humidity levels are maintained at well-defined levels.

  • What are the air temperature and humidity inside the International space station?
  • Have those levels been adjusted over the years?

A link to a page or document on some official website with this info will be appreciated.

Temperature and Humidity Control (THC) system is a part of the International Space Station's (ISS) Environmental Control and Life Support System (ECLSS). Thermal Control System (TCS) is a component part of the THC system and subdivides into the Active Thermal Control System (ATCS, PDF document) and Passive Thermal Control System (PTCS). From a PDF document on NASA facts:

Temperature and Humidity and Control subsystem helps maintain a habitable environment in the Station by removing heat and humidity, and circulating the cool dry air. Circulation of the atmosphere minimizes the temperature variations, ensures a well-mixed, breathable atmosphere and supports smoke detection;

   International Space Station's Environmental Control and Life Support System (ECLSS)

   Regenerative Environmental Control and Life Support System (ECLSS) Diagram (Source: NASA Science News)

The ECLSS is however a complex system and many of its parts failed on multiple occasions, for example the Oxygen Generation System (OGS) triggered several alarms, both the old one and its replacement that was activated in 2007. Now, this wasn't so critical because of redundancy in life support systems onboard the ISS, and e.g. the Russian segment has a Life Support Systems (LSS) too. So this should hopefully answer your question if the air temperature and humidity levels have ever been adjusted. With replacements and parts of individual systems failing these levels would obviously vary substantially at least on some segments of the ISS. And with many isolated systems, they would also vary segment to segment, module to module. Leaving the description of the complexity of all these systems to another document on ECLSS from the European Space Agency (ESA) from 2010 (also PDF):

Several Life Support systems are accommodated at the ISS to safely sustain its crew in conditions similar to those on the Earth. They are distributed amongst the ISS modules. Specific differences exist between the equipment located in the US and Russian Segment.

   ECLS Hardware

   On-Orbit ISS ECLS Hardware Distribution as of February 2010 (Source: NASA/ISEC)

And the document quotes these values:

  • Cabin pressure nominal range: T = 14.2 to 14.9 psi
  • Oxygen and nitrogen distribution: P = 93 - 120 psia, T = 60 - 113°F from Node 1
  • Oxygen Recharge: P = < 1050 psia (max 16 lb/h), T = 25 - 113 °F from PMA3
  • Nitrogen recharge: P = < 3400 psia (max 3 lb/h), T = 25 - 113 °F from PMA3
  • Cabin Temperature nominal range: T = 65 to 80˚F (18.3 to 26.7°C)
  • Cabin air velocity: 10 to 40 feet per minute
  • Atmospheric heat removal and humidity control: CCAA CHX
  • Airborne particulate and microbes removal and disposal: CCAA Filters
  • Waste water collection and distribution
  • Fuel cell water and potable water distribution
  • Pre-treated urine: 1 line from W&HC to WRS2, P < 5 psig, T = 65 - 105 °F
  • Process Water: 4 lines from WRS1 to WRS2
  • Cabin smoke detection: 2 Area smoke detectors
  • External venting lines: CO2 /CH4 , H2 , MCA, Cabin air
  • Air sampling: from Zen, Port, Stbd, Fwd ports to ARS rack (flow rate 100 - 400 scc/min)

Finding actual nominal performance quotes is a bit more tricky though with possibly many standalone systems active at the same time in different segments of the ISS. For example for the Russian segment, these are the quoted values:

Basic values of parameters of the atmospheric composition of the Russian segment, taking into account the gas exchange between the human body and the environment, fall within the following limits:

  • total (absolute) pressure in habitable pressurized compartments – 660-860 mm Hg;
  • ppO2 of inhaled air – 150-200 mmHg
  • ppCO2 – 6 mmHg, max.;
  • ppH2O – 10 ± 5 mmHg;
  • air temperature – 20-25°C (68-77°F);
  • air circulation rate – 0.1-0.4 m/sec (0.33-1.31 ft/sec);
  • noise level – 60 dB, max.;
  • contaminants – within maximum permissible concentrations as determined by the ГОСТ, the Russian contaminants standard

The humidity control is explained in the 4.2. Atmospheric Condensate Water Regeneration System section of that document and is a bit too complex to describe here, so I'll just recommend reading it. The document also quotes maximum and minimum values that trigger certain procedures, or alerts.

From another document by Kira Bacal, MD, PhD, MPH (direct Word file download) though that explains it in a bit more general sense (but is still 22 pages long):

The cabin environment is usually maintained at about 60% relative humidity (corresponding to approximately 0.2 psi of water vapor pressure).

I'll let others chip in with their findings on more exact values for other segment ECLS systems and their nominal performance. I wouldn't expect all of them to have the same requirements though, this is certainly not the case for scientific modules that would have stricter environmental control parameters, like for example is described in the sadly paywalled document on Centrifuge Accommodation Module (CAM) Cabin Air Temperature and Humidity Control Analysis (the module never made it to the ISS tho), so you'd be looking at a long list of per module, per sensor values.

Adding all the readings and changes in environmental temperature and humidity, intentional or accidental to the list would possibly stretch the limits of this Q&A, though. Just to give you a fair impression, these are some of the operational values for Node 1 Heater sensors alone, taken from International Space Station Thermal Control Group (TCG) Thermal Control System documentation (PDF):

Heater       Availability   Failure   Upper      Lower      Failure   Cyclic
(Sensor)                    Upper     Setpoint   Setpoint   Lower     Load
                            Limit                           Limit     Delta
1A (Snsr 1)  Enable BU      35.6      29.7       26.1       20.6      1.7
1A (Snsr 2)                 35.6      29.7       26.1       20.6      1.7
1B (Snsr 1)  Enable Opr     35.6      32.7       26.1       20.6      1.7
1B (Snsr 2)                 35.6      32.7       26.1       20.6      1.7
2A           Enable BU      35.6      26.1       23.3       17.8      1.7
2B           Enable Opr     35.6      26.1       23.3       17.8      1.7
3A (Snsr 1)  Enable BU      35.6      27.5       22.7       17.2      1.7
3A (Snsr 2)                 35.6      27.5       22.7       17.2      1.7
3B (Snsr 1)  Enable Opr     35.6      25.1       22.7       17.2      1.7
3B (Snsr 2)                 35.6      25.1       22.7       17.2      1.7
4A           Enable BU      35.6      25.5       22.7       17.2      1.7
4B           Enable Opr     35.6      25.5       22.7       17.2      1.7
5A (Snsr 1)  Enable BU      35.6      26.2       23.8       18.3      1.7
5A (Snsr 2)                 35.6      26.2       23.8       18.3      1.7
5B (Snsr 1)  Enable Opr     35.6      26.2       23.8       18.3      1.7
5B (Snsr 2)                 35.6      26.2       23.8       18.3      1.7
6A (Snsr 1)  Enable BU      35.6      26.8       24.4       18.9      1.7
6A (Snsr 2)                 35.6      26.8       24.4       18.9      1.7
6B (Snsr 1)  Enable Opr     35.6      26.8       24.4       18.9      1.7
6B (Snsr 2)                 35.6      26.8       24.4       18.9      1.7
7A (Snsr 1) Enable BU       35.6      28.5       22.7       17.2      1.7
7A (Snsr 2)                 35.6      28.5       22.7       17.2      1.7
7B (Snsr 1) Enable Opr      35.6      26.3       22.7       17.2      1.7
7B (Snsr 2)                 35.6      26.3       22.7       17.2      1.7
8A          Enable BU       35.6      24.9       22.2       16.7      1.7
8B          Enable Opr      35.6      24.9       22.2       16.7      1.7
9A          Enable BU       35.6      25.5       22.7       17.2      1.7
9B          Enable Opr      35.6      25.5       22.7       17.2      1.7

All Temperatures listed are in °C. To my knowledge, there is no archived or live telemetry data stream of all the sensor readings from the ISS available to public.

TL;DR - It's ... complicated. ;)

Update: Some of the ETHOS (Environmental and Thermal Operating Systems) Life Support, Thermal System and Regenerative Life Support data on The Space Station Live NASA's web page is finally available, so it should help you get an overview. It should be mentioned tho, that it doesn't include any telemetry data on the Russian segment of the ISS, and excludes large number of individual sensor readings, presenting per module current average values (with not all modules listed).

Here's an animation of all three pages (Life Support, Thermal System and Regenerative Life Support) of data available to public, valid for Nov 7, 2013 at 4:30 pm UTC:

   ETHOS data display from The Space Station Live

  • 2
    Circulation of the atmosphere also gives hints where to look for lost items, I've been told by Swedish astronaut Christer Fuglesang. – gerrit Oct 26 '13 at 12:04
  • 3
    Gotta love the TLDR. – a CVn Oct 26 '13 at 23:52
  • I am interested in the results of actual measurements inside the ISS, rather than nominal ranges of various climate control systems, although that info is also relevant and much appreciated. I am sure they have thermometers and humidity sensors inside the station. – Andrey Sokolov Oct 27 '13 at 4:54
  • 1
    @Andrey They have several dozen of sensors per each module, on the internal and external THC loops. As mentioned, there are also multiple standalone THC systems. Short of hacking into the ISS telemetry data stream, decrypting it and making sense of it all, I have no knowledge of how to get to actual live readings. I didn't find any archives of the readings available to the public either, short of what is quoted in numerous documents where environmental control systems are discussed. I've extended my answer and emphasised a few points a bit, which should cover it as much as it is possible here. – TildalWave Oct 27 '13 at 9:54

Quoting from the document posted at

Section III.1.2 Cabin Environment and EVA Environment.doc authored by Kira Bacal

(direct download link is provided in TildalWave's answer):

Ambient temperatures on current spacecraft are usually kept at “shirtsleeve” conditions, i.e. around 75°F.

The cabin environment is usually maintained at about 60% relative humidity.

It is not clear where these figures come from, how up-to-date they are, or whether they are applicable to the ISS.

(for reference: 75°F equals 24°C)

  • The document you mention from my answer is by Dr. Bacal that worked for several years at Johnson Space Center in the branch of Medical Informatics and Health Care Systems and is an active member of Space Medicine Associates. The document itself should cover its applicability well enough. The linked to chapters are from a pre-press version of Bacal, Beck, and Barratt, “Hypoxia, Hypercapnia, and Atmospheric Control”, in Textbook of Aerospace Medicine, ed. Barratt and Pool, Springer-Verlag. – TildalWave Oct 27 '13 at 10:15

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